A number of processes for preparing trialkyl phosphates have been known. These methods, as described in G. M. Kosolapoff, Organophosphorus Compounds, (John Wiley & Sons, Inc., New York, 1950), include: (1) the reaction of alkyl halides with metal phosphates; (2) the reaction of alcohols or phenols with phosphorus oxychloride in the presence of tertiary bases; (3) the reaction of phosphorus halides with sodium alkoxides or phenoxides; (4) the reaction of phosphorus oxychloride with an excess of an alcohol or phenol; and (5) the ring opening reactions of olefin oxides with phosphorus oxychloride. While each of these methods is efficacious, only the fourth method--the reaction of phosphorus oxychloride with an excess of alcohol or phenol--is commercially practiced due to economic considerations. In attempting to prepare a halogenated trialkyl phosphate, however, this method is generally ineffective because the phosphorus oxychloride will not by itself significantly react with a halogenated alcohol or phenol.
Economical and efficient methods of producing various halogenated compounds, for use as ignition resistant compositions or for use in such compositions, have been sought. The brominated compounds are known to be particularly useful in promoting ignition resistance. However, because of the bromine content these compounds tend to be unstable to ultraviolet and visible light. This tendency results in color changes, notably yellowing, that harm the cosmetic appearance of the final product into which the ignition resistance compound is incorporated and therefore decrease its commercial desirability. Compounds containing other halogens have similar tendencies.
The halogenated trialkyl phosphates, however, are known to impart ignition resistance while at the same time remain relatively stable to ultraviolet and visible light. Thus, when used in composition with a polymer, a reduced amount of discoloration will occur over time. At least one of these compounds, tris(tribromoneopentyl) phosphate, also acts as a plasticizer, thereby expediting processing when it is used in a composition of varying type. However, methods of preparing this compound and other halogenated trialkyl phosphates have, in the past, been expensive or time consuming, generally attaining only low conversion rates.
Thus, what is needed in the art is a process that is a simple, fast and high yield way to prepare halogenated trialkyl phosphate esters. This disclosure describes such an invention.